When coal is carbonized it first softens and, at or near the temperature of softening, decomposition begins. As the temperature increases, the coal becomes more and more fluid and the rate of decomposition also increases. It is thus evident that gaseous decomposition products are being formed in a fluid mass. The degree of fluidity of this system together with the rate of formation of the gaseous products determines in large measure the amount of pressure that is exerted against the walls that confine the coal. This system is one of the sources of the pressure. There are two conditions, however, that act to relieve the pressure.
These conditions are: (a) the void spaces between the coal particles, and (b) the shrinkage of the newly-formed coke as the temperature of the mass increases. The former is inversely related to the bulk density of the mass of coal particles; for, the more void spaces present, or the lower the bulk density, the more readily can the softening particles of coal expand into the void spaces.
There is another source of pressure that is also due to the formation of gases but the mechanism is somewhat different. It is related not only to the rank of the coal and its consequent fluid properties but it is also related to the coke oven itself. In a coke oven two plastic seams parallel to the oven walls are established soon after the coal is charged and these progress toward the center of the oven as carbonization proceeds. Because there is considerable heat stored in the roof, floors, and doors, plastic seams are also started from these surfaces and progress away from them at a rate proportional to the heat available. In other words, there is a continuous plastic envelope around the uncarbonized coal from soon after the time the coal is charged until the two plastic seams starting from the walls, meet at the center of the oven. Gases evolved into the raw coal side of the plastic seams are therefore entrapped within the plastic envelope. At or near the end of the carbonization period or when the two plastic seams are about to meet at the center of the oven, with the envelope still entrapping the gas, the rate of heating increases sharply thus increasing the amount of gas evolved with the net result that the pressure rises rapidly. As soon as the two plastic seams have met and solidified into coke, a rapid decrease in pressure occurs due to the disappearance of the envelope. While sawdust has been previously found to reduce carbonization pressure (C. C. Russell, M. Perch, J. F. Farnsworth, Reducing Coal-Expansion Pressure, AIME Proc. Blast Furnace, Coke Oven, Raw Materials Conference, 8, 32-50 (1949), the mechanism has been by the reduction of coal bulk density which is known can be reduced by several means including moisture addition and finer pulverization.
As shown in FIG. 7 of the above publication, loose sawdust did not eliminate the pressure peak near the end of the coking period with 4 and 6% sawdust addition, and it was only with 8% addition, and a substantial decrease in bulk density to 41.3 lbs. per cu. ft. where the peak disappeared. The lowering of bulk density means loss in coke oven productivity as well as serious effect on coke strength.
Sawdust and other materials in the form of flakes however, straddle the plastic seams in the oven charge to provide the necessary passage for the gas and disruption of the plastic envelope without necessarily decreasing the bulk density, oven productivity, nor affecting the properties of the resultant coke as is the case with loose sawdust addition.